The mechanisms by which supernumerary sperm are excluded from mammalian eggs will be studied in the golden hamster, which exhibits a strong zona reaction. Specific attention will be given to elucidating the role of the cortical granules in the prevention of polyspermy. The origin and cytochemical composition of these granules will be determined by ultrastructural studies. Inhibitors of cortical granule release will be used to determine whether cortical granule dehiscence is required for monospermic fertilization. Fertilization will be studied in vitro to define the temporal relationships of cortical granule dehiscence, the establishment of a block to polyspermy at the zone pellucida and at the oolemma and also to determine whether a rapid block to polyspermy exists in mammals. Various membrane probes, visible with the electron microscope, will be used to monitor changes in membrane characteristics which occur subsequent to fertilization and which might contribute to a rapid block to polyspermy. These studies should provide information on rapid changes in membrane configuration, as well as elucidate how membranes are altered to prevent further fusion. Futhermore, they should contribute to our understanding of mammalian blocks to polyspermy.